1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly, to a liquid crystal display apparatus of a front light type.
2. Description of the Related Art
A liquid crystal display apparatus is widely used as a monitor of office automation equipment for the reason of small size, thin shape and low power consumption. Such a liquid crystal display apparatus is composed of a liquid crystal display panel, an optical section and a housing. In the liquid crystal display panel, liquid crystal is put between opposing substrates. The orientation of liquid crystal molecules is controlled using electric field generated based on voltage applied between electrodes provided for substrates or one of the substrates. The optical section is composed of optical members such as a light source, a light guide plate, and a light spreading board. The housing supports the liquid crystal display panel and the optical section.
Also, the liquid crystal display apparatus is used as a display unit of a mobile terminal equipment such as a portable phone and a PDA for the reasons of thin shape and light weight. However, in the mobile terminal equipment, an amount of data to be handled increases for the purpose of display of Internet data. For this reason, the development for small size has been preceded, whereas expansion of a display area is demanded. To respond to such a demand, improvement has been carried out in not only the structure of the liquid crystal panel but also the assembling method of the light source and the optical members.
As methods of illuminating the liquid crystal display panel, two methods are known, i.e., a backlight method in which light is irradiated from the back side of the liquid crystal panel and a display image is viewed as the transmission light of the liquid crystal panel. The other method is a front light method in which light is irradiated from a light guide plate provided for a front portion of the liquid crystal panel and a display image is viewed as reflection light from the liquid crystal panel. In the above mobile terminal equipment, one of these two methods is mainly adopted.
In this front light method, a light source is provided along the side ends of the light guide plate. The liquid crystal display panel and the light guide plate are fixed in parallel with a predetermined gap using an adhesive material layer in order to control the direction of the light. The light from the light source is led into miniature unevenness provided for the light guide plate on the viewing side and the light reflected from the surface of the liquid crystal panel is outputted to the viewer.
With the fixation of the liquid crystal panel and the light guide plate, there are two methods: one method in which the whole surface of the liquid crystal panel is fixed with the adhesive material layer, and the other method in which the peripheral region outside the liquid crystal panel is fixed with the adhesive material layer. The former is known in Japanese Laid Open Patent Application (JP-P2002-23155A). In this conventional example, air is between the light guide plate and the liquid crystal panel and reflection increases in the boundary between the light guide plate. As a result, the reflection light overlaps an original display image on the liquid crystal display panel and occurs a white zone and the decrease of contrast. That is, the above conventional example has the purpose of prevention of the white zone and the decrease of contrast. In order to achieve the purpose by preventing the reflection at the boundary between the light guide plate and the air in the display area, an adhesive material layer is provided for the whole portion between the light guide plate and the liquid crystal display panel. Also, relation of the refractive index of the adhesive material and the refractive index of the light guide plate is restricted such that the refractive index of the light guide plate is 1.49 or more and the refractive index of the adhesive material is 1.48 or less. Thus, the light with a large incident angle is perfectly reflected at the boundary between the adhesive material layer and the light guide plate to prevent that the light with the large incident angle is incident on the display area.
In the above-mentioned method of fitting the liquid crystal display panel and the light guide plate in the whole portion, the light outputted from the light source propagates the adhesive material layer twice. Therefore, the brightness decreases because the absorption of the light by the adhesive material layer is large even if the adhesive material with a high transmittivity is used. Also, if a light quantity of the light source is increased to improve the brightness, the size and power consumption of the light source become large. Therefore, the method is not desirable for a liquid crystal display apparatus for the mobile terminal equipment.
From the above reasons, in the mobile terminal equipment, a method is used of fixing a light guide plate and a liquid crystal display panel in a peripheral area outside the display area. The liquid crystal display apparatus of this structure will be described with reference to FIGS. 1A and 1B. FIG. 1A is a cross sectional view showing the structure of the conventional liquid crystal display apparatus of a front light type in the neighborhood of the display area. FIG. 1B is an expanded view of a circled portion in FIG. 1A.
As shown in FIGS. 1A and 1B, the liquid crystal display apparatus of the front light type is composed of a liquid crystal display panel 102, a light source 4, a light guide plate 103 and a housing 101. In the liquid crystal display panel 102, liquid crystal is put between opposite substrates. The light source emits light for illuminating the liquid crystal panel 102. The light guide plate 103 guides the light emitted from the light source 104 to the side of the display area 108, reflecting the light by a small unevenness provided for a view side and leads into the liquid crystal panel 102. The housing 101 fixes them. Also, the opposing surfaces of the liquid crystal panel 102 and the light guide plate 103 are both formed to be flat. However, they are not perfectly flat. Therefore, a Newton's ring is seen in a contact section when both are fit. For this reason, a space holding member 105 such as a two sided tape which has an adhesive material layer 109 on both sides is provided for purposes of fixation of front light, the improvement of impact endurance, and holding the gap between the liquid crystal panel 102 and the light guide plate 103.
As the material of the adhesive material layer provided for one or two sides of the space holding member 105, acrylic adhesive material or silicon adhesive material is generally used, whereas the light guide plate 103 is also often formed of acrylic material. Therefore, the refractive index of the light guide plate 103 and the adhesive material 109 is substantially the same. The light incident from the light guide plate 103 goes inside the adhesive material layer 109 without being perfectly reflected in the surface of the adhesive material layer 109, and propagates to the end of the adhesive material layer 109 through dispersion, absorption, and scattering, as shown in FIGS. 1A and 1B.
The space holding member 105 such as the two sided tape is formed by cutting original tape in a predetermined size. The adhesive material layer 109 extends from a base member 110 in the outer direction to the extent of 20 to 30 μm or more in the manufacture. When the light incident on the adhesive material layer 109 reaches the extending portion 111 of the adhesive material layer 109, the light is outputted from the extending portion 111 to illuminate like points, a line or a zone. This illumination is referred to as a bright zone in this specification.
In case of the liquid crystal display apparatus used for the monitor of the OA equipment and so on, there is not so severe limitation on the size of the equipment. Therefore, the space holding member 105 can be separated from the tip section of the display area 108 or housing 101 and can be formed in withdrawing in the back. However, in case of the equipment such as the mobile terminal equipment in which a small size and a large display screen are required, it is not possible to increase the distance (W in FIG. 1B) between the extending portion of the adhesive material layer 109 and the tip section of housing 101. When the liquid crystal display apparatus is viewed in a slant direction, the extending portion of the adhesive material layer 109 functions as the bright zone and the display quality degrades remarkably.
Also, because the refractive index of the adhesive material layer 109 is substantially the same as that of the light guide plate 103, the most of the light incident in the direction of the space holding member 105 reaches the adhesive material layer 109. Therefore, the quantity of the light transferred to the distal portion of the light guide plate 103 decreases. As the result, efficiency of use of the light decreases, and the brightness decreases and brightness uniformity is deteriorated.
Also, in the conventional structure, a two sided tape which the adhesive material layer 109 has stuck out is determined not to be good for the purpose of the bright zone. Therefore, the manufacturing yield of the two sided tape becomes low, and the cost of the liquid crystal display apparatus has risen.